1. Field of the Invention
The present invention relates to a universal joint having two degrees of rotational freedom or three degrees of freedom in rotation and a parallel mechanism machine incorporating the universal joint.
2. Description of the Related Art
A spherical slide bearing and a rolling bearing are known as universal joint having three degrees of freedom in rotation. FIG. 7 is a diagram showing the structure of a spherical slide bearing which incorporates a node 1 having an end to which a semi-spherical socket 2 is joined. A sphere 4, to which a node 5 is joined, is fitted into the socket 2 such that separation of the sphere 4 is inhibited by a cover 3. Inclination of the sphere 4 in all directions within the socket 2 in a certain range is permitted. As a result, the spherical slide bearing serves as a universal joint having the three degrees of freedom in rotation.
As a similar spherical slide bearing, a bifurcated spherical slide bearing is known which incorporates either node thereof which is bifurcated. FIG. 8 is a diagram showing the structure of the bifurcated spherical slide bearing. Similarly to the spherical slide bearing, a node 1 is joined to a socket 2. A node 8 is joined to the left-hand semi-sphere 6, while a node 9 is joined to the right-hand semi-sphere 7. The two semi-spheres 6 and 7 are held by the socket 2 and a cover 3 similarly to the sphere 4 of the above-mentioned spherical slide bearing.
As a result of the foregoing structure, each of the two semi-spheres 6 and 7 is able to rotate perpendicularly to its plane and around an axis which passes through the center of the circle. Therefore, the semi-spheres 6 and 7 held in the socket 2 can be inclined in all directions.
As described above, the bifurcated spherical slide bearing is able to act as a universal joint having three degrees of freedom in rotation, similarly to the above-mentioned spherical slide bearing.
FIG. 6 shows a parallel mechanism machining tool incorporating a universal joint of the foregoing type. The parallel mechanism is a mechanism incorporating a plurality of links which connect a movable member and stationary members to one another. Thus, the parallel mechanism is able to control the position and the attitude of the movable member. A variety of machines, such as machining tools, industrial machines and robots, each incorporating the above-mentioned mechanism are in existence. Referring to FIG. 6, reference numerals 36 and 37 represent universal joints each of which is the universal joint shown in FIG. 7. The number of the universal joints 37 is the same as the number of the links (nodes) 38. Note that a structure is sometimes employed in which one universal joint 37, which is the universal joint shown in FIG. 8, is provided for two links (nodes) 38.
The above-mentioned universal joint which has a simple structure suffers from looseness, unsatisfactory accuracy and insufficient rigidity because the above-mentioned universal joint is the slide bearing. What is worse, there arises a problem in that heat is easily produced due to excessive friction. The structure of the foregoing universal joint that the spheres are secured encounters unsatisfactorily narrow movable range.
On the other hand, the rolling bearing is able to prevent looseness when a previous pressure is applied to the rolling bearing. As a result, the accuracy can be improved and the rigidity can be increased. Moreover, friction can be reduced and, therefore, the problem of production of heat can be prevented. Therefore, a universal joint structured as shown in FIG. 9 and having three degrees of freedom in rotation is known. The universal joint shown in FIG. 9 is constituted by combining three rolling bearings with one another as a substitute for the spherical slide bearing. The foregoing universal joint suffers from a problem of a narrow movable range and a necessity for a large size if required rigidity is obtained. In the structure shown in FIG. 9, the rolling bearing is disposed in a portion in which a node 1 and a U-shape base 40 are connected to each other, a portion in which the U-shape base 40 and a rectangular rotary frame 41 are connected to each other and portions in which the rotary frame 41 and two nodes 8 and 9 are connected to one another.
The parallel mechanism machine incorporating the universal joint shown in FIG. 7 or that shown in FIG. 8 is a slide bearing. Therefore, accuracy and rigidity required for the machine cannot be maintained. If an attempt is made that sufficient rigidity is maintained, the size of the joint must be enlarged. What is worse, inevitable excessive friction causes a problem to arise in that required durability cannot be obtained. Also the universal joint constituted by combining the rolling bearings each of which is shown in FIG. 9 encounters a problem of a narrow movable range which arises due to unsatisfactorily narrow movable range of the joint. Also the size of the joint is enlarged undesirably when satisfactory great rigidity is attempted to be maintained. If the size of the joint is enlarged, there arises a problem in that the original characteristic of the parallel mechanism machining tool that the movable member is a light member cannot be realized because the universal joint shown in FIG. 6 has the structure that the universal joint 37 is disposed adjacent to the movable member 39.
Accordingly, an object of the present invention is to provide a universal joint having two or three degrees of freedom in rotation and exhibiting a wide movable range, great rigidity, satisfactory accuracy and small size.
Another object of the present invention is to provide a parallel mechanism machine incorporating a universal joint exhibiting small size, great rigidity and a wide movable range.
To achieve the foregoing object, according to a first aspect of the present invention, there is provided a universal joint comprising: two bearing portions to permit rotation of the universal joint in two axial directions, wherein centers of rotation of the two bearing portions substantially coincide with each other, the bearing portions have axes of rotation which are substantially perpendicular to each other, and at least one of the two bearing portions is a circular-arc guide bearing.
According to a second aspect of the present invention, there is provided a universal joint comprising: three bearing portions to permit rotation of the universal joint in three directions, wherein two bearing portions in two axial direction of the three axial directions have centers of rotation which substantially coincide with each other, at least either of the two bearing portions is a circular-arc guide bearing, and the bearing portion in the residual axis of the three axes has an axis of rotation which passes through substantially the centers of rotations of the two axes and which is substantially perpendicular to the two axes.
According to a third aspect of the present invention, there is provided a universal joint having a structure according to the first aspect or the second aspect, further comprising a bifurcated link mechanism connected to the bearing portion or the circular-arc guide bearing and having centers of rotation which substantially coincide with each other.
According to a fourth aspect of the present invention, there is provided a universal joint having a structure according to the any one of first to third aspects, wherein the circular-arc guide bearing is a circular-arc rolling guide.
According to a fifth aspect of the present invention, there is provided a parallel mechanism machine comprising a universal joint according to any one of first to fourth aspects.
Other objects, features and advantages of the invention will be evident from the following detailed description of the preferred embodiments described in conjunction with the attached drawings.